What are the VarTypes... for in AddLambda()?

TBaseMulticastDelegate has a function signature

template<typename FunctorType, typename... VarTypes>
inline FDelegateHandle AddLambda(FunctorType&& InFunctor, VarTypes... Vars)

What are the Vars used for?

Like the other Add* functions, they let you curry extra parameters to be passed back to your lambda when the delegate is invoked.

just pass the parameters, for example:
createlambda(&urmodule::urfunc,1,2,3)